Re: [Xen-devel] [PATCH v3 07/15] argo: implement the register op

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jan 09, 2019 at 12:02:34PM -0500, Julien Grall wrote:
> Hi,
> 
> Sorry for the formatting. Sending it from my phone.
> 
> On Wed, 9 Jan 2019, 11:03 Christopher Clark, <christopher.w.clark@xxxxxxxxx>
> wrote:
> 
> > On Wed, Jan 9, 2019 at 7:56 AM Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> > >
> > > On Sun, Jan 06, 2019 at 11:42:40PM -0800, Christopher Clark wrote:
> > > > The register op is used by a domain to register a region of memory for
> > > > receiving messages from either a specified other domain, or, if
> > specifying a
> > > > wildcard, any domain.
> > > >
> > > > This operation creates a mapping within Xen's private address space
> > that
> > > > will remain resident for the lifetime of the ring. In subsequent
> > commits,
> > > > the hypervisor will use this mapping to copy data from a sending
> > domain into
> > > > this registered ring, making it accessible to the domain that
> > registered the
> > > > ring to receive data.
> > > >
> > > > Wildcard any-sender rings are default disabled and registration will be
> > > > refused with EPERM unless they have been specifically enabled with the
> > > > argo-mac boot option introduced here. The reason why the default for
> > > > wildcard rings is 'deny' is that there is currently no means to
> > protect the
> > > > ring from DoS by a noisy domain spamming the ring, affecting other
> > domains
> > > > ability to send to it. This will be addressed with XSM policy controls
> > in
> > > > subsequent work.
> > > >
> > > > Since denying access to any-sender rings is a significant functional
> > > > constraint, a new bootparam is provided to enable overriding this:
> > > >  "argo-mac" variable has allowed values: 'permissive' and 'enforcing'.
> > > > Even though this is a boolean variable, use these descriptive strings
> > in
> > > > order to make it obvious to an administrator that this has potential
> > > > security impact.
> > > >
> > > > The p2m type of the memory supplied by the guest for the ring must be
> > > > p2m_ram_rw and the memory will be pinned as PGT_writable_page while
> > the ring
> > > > is registered.
> > > >
> > > > xen_argo_page_descr_t type is introduced as a page descriptor, to
> > convey
> > > > both the physical address of the start of the page and its
> > granularity. The
> > > > smallest granularity page is assumed to be 4096 bytes and the lower
> > twelve
> > > > bits of the type are used to indicate the size of page of memory
> > supplied.
> > > > The implementation of the hypercall op currently only supports 4K
> > pages.
> > > >
> > >
> > > What is the resolution for the Arm issues mentioned by Julien? I read
> > > the conversation in previous thread. A solution seemed to have been
> > > agreed upon, but the changelog doesn't say anything about it.
> >
> > I made the interface changes that Julien had asked for. The register
> > op now takes arguments that can describe the granularitity of the
> > pages supplied, though only support for 4K pages is accepted in the
> > current implementation. I believe it meets Julien's requirements.
> 
> 
> I still don't think allowing 4K or 64K is the right solution to go. You are
> adding unnecessary burden in the hypervisor and would prevent optimization
> i the hypervisor and unwanted side effect.
> 
> For instance a 64K hypervisor will always map 64K even when the guest is
> passing 4K. You also can't map everything contiguously in Xen (if you ever
> wanted to).
> 
> We need to stick on a single chunk size. That could be different between
> Arm and x86. For Arm it would need to be 64KB.

Doesn't enforcing 64KB granularity has its own limitation as well?
According to my understanding of arm (and this could be wrong), you
would need to have the guest allocate (via memory exchange perhaps) 64KB
machine contiguous memory even when the hypervisor doesn't need it to be
64KB (when hypervisor is running on 4KB granularity).

I think having a method to return granularity to guest, like Stefano
suggested, is more sensible. Hypervisor will then reject registration
request which doesn't conform to the requirement.

Wei.

> 
> Cheers,
> 
> 
> > thanks,
> >
> > Christopher
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxxx
> > https://lists.xenproject.org/mailman/listinfo/xen-devel

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